Monoclonal antibodies are the most widely used class of therapeutic proteins when high affinity and specificity for a target molecule are desired. However, non-antibody proteins that can be engineered to bind such targets are also of high interest in the biopharmaceutical industry. These “alternative scaffold” proteins may have advantages over traditional antibodies due to their small size, lack of disulphide bonds, high stability, and ability to be expressed in prokaryotic hosts. Novel methods of purification are readily applied; they are easily conjugated to drugs/toxins, penetrate efficiently into tissues and are readily formatted into multispecific binders (Skerra 2000; Binz and Pluckthun 2005).
One such alternative scaffold is the immunoglobulin (Ig) fold. This fold is found in the variable regions of antibodies, as well as thousands of non-antibody proteins. It has been shown that one such Ig protein, the tenth fibronectin type III (FN3) repeat from human fibronectin, can tolerate a number of mutations in surface exposed loops while retaining the overall Ig-fold structure. Thus, libraries of amino acid variants have been built into these loops and specific binders selected to a number of different targets (Koide et al. 1998; Karatan et al. 2004). Such engineered FN3 domains have been found to bind to targets with high affinity, while retaining important biophysical properties (Parker et al. 2005).
Desirable physical properties of potential alternative scaffold molecules include high thermal stability and reversibility of thermal folding and unfolding. Several methods have been applied to increase the apparent thermal stability of proteins and enzymes, including rational design based on comparison to highly similar thermostable sequences, design of stabilizing disulfide bridges, mutations to increase α-helix propensity, engineering of salt bridges, alteration of the surface charge of the protein, directed evolution, and composition of consensus sequences (Lehmann and Wyss 2001). High thermal stability is a desired property of such scaffolds as it may increase the yield of recombinant protein obtained, improve solubility of the purified molecule, improve activity of intracellular scaffolds, decrease immunogenicity, and minimize the need of a cold chain in manufacturing.